1. Field of the Invention
This invention relates to displays which have special utility in connection with attachments for helmets worn by aircraft pilots (helmet mounted displays) wherein an image of a desired display, such as information on the face of a cathode ray tube, can be introduced into the field of view being observed by the pilot so that the desired display is superimposed upon the scene being viewed in the pilot's line of sight.
2. Description of the Prior Art
Helmet mounted displays are well-known in the prior art and usually employ an optical system which receives an image of a desired display from a cathode ray tube and projects a collimated image thereof using a partially reflective surface, so that the image is viewed at infinity and is combined with the pilot sees through the aircraft windshield. In a U.S. Pat. No. 4,859,030 of Donald J. Rotier issued Aug. 22, 1989 and assigned to the assignee of the present invention, a helmet mounted display is shown wherein a combiner is mounted on the helmet in front of one of the pilot's eyes and includes a curved surface and a filter that receives light from an image to be superimposed on the scene being viewed by the operator through the windshield. The Rotier apparatus, although generally quite satisfactory, has several drawbacks by virtue of the combiner structure being attached to the helmet relatively close to the pilot's eye. This proximity to the eye may present safety hazards to the pilot in the event that the helmet is knocked forward on his head. Furthermore, the optical elements utilized in the path from the cathode ray tube to the combiner are rather complicated and some elements are off center causing them to be very difficult to manufacture. Furthermore, being a monocular, both eyes do not see the same scene and, in the event of a cathode ray tube failure, the entire display is lost. Also, in the Rotier device, the apparatus mounting the combiner is close to the eye and gives obstruction to some directions of viewing.
Another prior art helmet mounted display is shown in FIGS. 1 and 2 of the present disclosure. In this prior art apparatus, a cathode ray tube shown generally by reference numeral 10 is mounted to the top portion of the helmet 12 of a pilot. A rather complex arrangement of optical elements, better seen in FIG. 2, operates as a relay for directing the image from the source 10 to a pair of reflecting surfaces 14 and 16 and then sidewise to a second pair of reflecting surfaces 18 and 20 where the images are reflected downwardly to a pair of partially reflecting surfaces 22 and 24. Reflected light from surfaces 22 and 24 is thereafter directed to the pupils 26 and 28 of the pilot's eyes and he views the desired image superimposed on his view of the scene through the surfaces 22 and 24. While this apparatus does provide dual images of the display being projected, it does not solve the problem of a cathode ray tube failure since only one is still used and, furthermore, because the optics has to be mounted high on the helmet, it presents a secondary hazard to the pilot in the event of ejection. The weight of the optical relay system on the top of the helmet produces a high center of gravity for the helmet on the pilot's head which, during ejection, could cause forces against the pilot's neck that could even result in breakage thereof. Another major problem encountered with the prior art apparatus of FIGS. 1 and 2 is the joining line between the two partially reflective surfaces 22 and 24 identified in FIG. 2 as a cusp 30. Cusp 30 obviously provides extremely undesirable distortion when the pilot attempts to look through the area of the cusp.